SAC Phase 1 Analytical Studies of Building Performance
Project Title:
Analysis of a Six Story Steel Moment Frame Building in Santa Monica
Sub-contractors:
Michael D. Englehardt, University of Texas, Austin
Keedon Kim, University of Texas, Austin
Thomas A Sabol, Englekirk & Sabol Consulting Engineers, Inc.
Lawrence Ho, Englekirk & Sabol Consulting Engineers, Inc.
Hae-In Kim, Englekirk & Sabol Consulting Engineers, Inc.
Joseph Uzarski, SOH Associates
Abunnasr Husain, SOH Associates
Project Summary:
This report summarizes the results of an analytical study of a six-story steel moment frame
building located in Santa Monica, California. This study was conducted as part of the SAC Task
3.1. This particular building is identified under SAC Task 3.1 as Building Site 7.
This building sustained significant damage to its steel moment frame joints in the 1994
Northridge earthquake. The primary form of damage observed in this building was the fracture
of beam flange welds, typically at the beam bottom flange. A large number of the buildings
120 moment frame joints experienced some damage.
Analytical studies were conducted to investigate the predicted response of the building in the
Northridge earthquake, and its predicted response under other strong ground motion records. Three
types of analyses were conducted in this study: two-dimensional elastic analysis,
three-dimensional elastic analysis, and two-dimensional inelastic analysis. A principal objective
of this study was to determine if the damage observed in this building was in any way predictable
by structural analysis, and if structural analysis could be used as a tool to guide building
inspections.
The primary gound motion record used in the analysis of this building was the Santa Monica
City Hall record from the 1994 Northridge earthquake. The record site is very close to the
building site, and this record was considered the best estimate available of the actual ground
motion experienced by the building during the Northridge earthquake. In addition to the Santa
Monica City Hall record, a variety of other strong motion records were used in the analysis. This
included other records from the Northridge earthquake, as well as records from the other strong
earthquakes
The results of both the elastic and inelastic analyses using the Santa Monica City Hall record
showed some degree of correlation between the predicted structural response and the damage
observed at the joints. There were also numerous exceptions to this trend. Nonetheless, the
results of this study suggest that an elastic structural analysis could have been used as a useful
guide for inspecting this building. It appears that the chances of locating a damaged joint
would be increased by first increasing joints with the highest predicted beam moment demand
capacity ratios (DCRs). For joints with similar levels of beam moment DCR, the chances of
locating a damaged joint would be increased by first inspecting the joints for the heaviest
beams.
The inelastic analysis indicates that the maximum beam plastic rotations experienced in this
building during the Northridge earthquake were on the order of 0.010 rad. These rather modest
inelastic deformation demands suggest that the moment frame joints performed quite poorly.
For the other ground motions considered in this study, the analyses predict significantly
greater structural demands on this building (beam moment DCRs, beam plastic rotations, interstory
drift ratios, etc.) than were predicted using the Santa Monica City Hall record. Much higher
levels of damage might be expected in this building under these other strong ground motions, than
was experienced in the Northridge earthquake. The simulated Elysian Park record appeared to be
particularly damaging to this building.
Excluding the Elysian Park record, the maximum beam plastic rotations developed in this
structure under a variety of very strong gound motions were on the order of 0.02 to 0.03 rad.
The Elysian Park record developed a maximum beam plastic rotation of about 0.04 rad.
No attempt was made in this study to model the hysteretic response of a damaged joint.
Therefore, no conclusions can be drawn on the consequences of connection damage on the response of
this structure in future strong earthquakes.
